[(230000/(1+r )^13)*(1+r)^5 +18433]*(1+7/100)^8=230000 subst. x=r+1
[(230000/(x)^13)*(x)^5 +18433]*(1+7/100)^8=230000
x^8 = 3951828213613416230000/1983286741471582167967 mit x!=0
r^8+8 r^7+28 r^6+56 r^5+70 r^4+56 r^3+28 r^2+8 r = 1968541472141834062033/1983286741471582167967
r1 = -1-107*(23/1983286741471582167967)^{1/8}*sqrt(10) = -2.090000177380988652235974...
r2 = 107*(23/1983286741471582167967)^{1/8} sqrt(10)-1 = 0.09000017738098865223597432...
komplexe Lösungen lasse ich mal weg.